A variety of configurations of electrode arrays, which are known in the art, are useful for monitoring electrical activity of the heart by generating electrocardiograms (ECG's), which characterize cycles of cardiac muscle contraction and relaxation as myocardial cells depolarize and then repolarize. Those skilled in the art are familiar with limb leads and precordial leads, which are attached to a patient's skin for generating far-field ECG's, as well as electrode arrays that are included in subcutaneously implantable cardiac monitoring devices and systems for the same purpose.
FIG. 1A is a schematic diagram showing an array of subcutaneously implanted cardiac monitoring electrodes 110, which are configured to create a pair of sensing vectors. FIG. 1B shows portions of two exemplary ECG waveform plots N and I, which may be generated from electrical cardiac signals as sensed by electrodes 110, wherein the sensed signals are projections of the electric force vectors (corresponding to the aforementioned depolarization and repolarization) on each sensing vector of electrodes 110. ECG N represents a typical cardiac cycle, wherein atrial contraction is represented by a P wave, ventricular depolarization by a QRS complex, and ventricular repolarization by a T wave. ECG I represents a cardiac cycle when a portion of the heart muscle may be ischemic as indicated by a deviation in the ST segment. (The ST segment and the deviation therefrom are both indicated with a bold arrow in each plot of FIG. 1B.) A pair of sensing vectors, arranged approximately orthogonal to one another, is preferred in order to reliably detect ST segment deviations, which may be indicative of ischemia in instances of acute myocardial infarction.
FIGS. 2A-B are plan views of two exemplary, prior art implantable medical devices 210, 230, each of which include electrodes 10 arranged in an array similar to that illustrated in FIG. 1A, so as to create a pair of sensing vectors. FIG. 2A illustrates device 210 including a hermetically sealed housing, or can 212, which contains electronic circuitry and an associated power source (not shown), a connector module 214, by which one or more elongate implantable medical electrical leads can be coupled to the circuitry contained within can 212, and electrodes 10 mounted on can 212 or module 214. Those skilled in the art are familiar with implantable pulse generators for cardiac stimulation and sensing, such as device 210, which are typically implanted in a pectoral subcutaneous pocket with the associated electrical leads extending therefrom to either an endocardial location, or an epicardial location. Although the associated electrical leads carry electrodes that may used for monitoring electrical activity of the heart, a far-field ECG generated by subcutaneously located electrodes 10 can provide useful information in a variety of applications. FIG. 2B illustrates device 230, which, like device 210, includes a hermetically sealed housing, or can 232, in which electronic circuitry and a power source are contained, and electrodes 10, all of which are mounted on can 232. Device 230 may be employed solely for cardiac monitoring via electrodes 10 that generate a far-field ECG, for example, from a subcutaneous implant location like that illustrated in FIG. 1A, which is accessed via an incision 15. Those skilled in the art are familiar with other forms of subcutaneously implantable cardiac monitors, for example, the Reveal® device manufactured by Medtronic, Inc. of Minneapolis, Minn.
Recent advances in the fabrication of electronics and associated power sources allow for a significant volume reduction of subcutaneous cardiac monitoring devices, for example, allowing for minimally invasive implant through a relatively small incision. Yet, the smaller volume may not accommodate the necessary spacing of electrodes to create a preferred arrangement of a pair of sensing vectors.